The invention relates to a process for the replacement of guide pins of a guiding tube forming part of the top internal equipment of a pressurised water nuclear reactor.
In pressurised water nuclear reactors the top internal equipment of the vessel is disposed above the reactor core, inside the vessel. This top internal equipment includes vertical guiding tubes of great length which permit the guiding of the control rods. These guiding tubes are disposed vertically in line with certain assemblies of the core, in which the control rods are moved in order to control the reactivity of the reactor. These tubes also guide the control rods when they fall, through the action of gravity, to the maximum insertion position inside the assemblies in the event of an emergency shut-down of the reactor.
The guiding tubes are disposed inside a structure consisting of two horizontal plates, one of which, at the bottom of the internal equipment, is composed of the top core plate while the other consists of the support plate for the top internal equipment. These two plates are connected by braces and are provided with apertures for the passage of the guiding tubes, these apertures being in alignment with one another.
The guiding tubes are composed of two independent parts in line with one another in the axial direction, that is to say the vertical direction.
The lower part of these guiding tubes, which comprise in particular the continuous guiding means for the control rods, rests by its bottom end on the top core plate. On the flange constituting the base of this bottom end are fixed guide pins which have a downwardly projecting resilient deformation portion which engages in apertures provided in the top core plate.
The guiding tubes are thus held in a fixed position relative to the top plate and to the reactor core assemblies.
The lower part of the guiding tubes also carries at its top end a flange intended to rest on the support plate for the internal equipment.
The upper part of the guiding tube has at its bottom end a flange which comes to rest on the support plate of the internal equipment, above the flange of the lower part of the guiding tube. The connecting bolts enable the two parts of the guiding tube to be connected to the support plate and to be connected to one another in such a manner as to be in line with one another.
During the refuelling of the nuclear reactor with fuel assemblies and during certain maintenance or repair operations on the reactor, the whole of the top internal equipment is removed from the vessel while the reactor pool and vessel are filled with water, and is disposed in a storage bay in the nuclear reactor pool.
Various inspection and maintenance operations for the internal equipment are possible when this internal equipment is disposed in its storage bay.
In particular, in inspection operations for the top internal equipment, use is made of an inspection machine comprising an ultrasonic detector which can be moved under the internal equipment, at the bottom of the reactor pool, in order to check the guide pins disposed inside the openings in the top plate of the reactor core (Patent FR-A-No. 2.495.816).
It has thus been possible to discover certain defects in guide pins, which may necessitate their replacement.
Hitherto, no processes or apparatus capable of permitting replacement of these guide pins have been known.
The operations of replacement of the guide pins entail, in fact, the complete dismantling of these guide pins, and in particular the complete elimination of the portion of the pin which remains engaged in the bottom flange of the guiding tube.
In certain cases this elimination of the remaining portion of the guide pin can be achieved only by electroerosion.
Moreover, the fastening of a new guide pin requires certain welding operations.
Consequently, these operations cannot be carried out under water in the top internal equipment storage bay.
Furthermore, after a certain period of operation the guiding tubes are contaminated by radioactive products, which prevent any direct intervention on the guiding tube without adequate biological protection or without thorough decontamination of the guiding tube.
Dismantling of the guiding tube entails, in fact, the separation of the two parts of which it is composed, since the connecting bolts join the two parts of the guiding tube together and fasten it on the internal equipment support plate.
When the two parts of the guiding tube are refitted in line with one another, it is very difficult to find once again the absolutely exact previous relative position of the two parts of the guiding tube.
This may result in misalignment, which is increased by the fact that the wear on the guide cards and on the continuous guiding elements of these tubes is not absolutely regular.
After the guiding tubes have been reinstalled, there is therefore still a risk that the internal equipment may no longer permit perfect guiding of the control rods, which is very particularly troublesome in respect of the dropping of these rods under their own weight in the event of an emergency shut-down.